In recent years, as a radical solution of energetic and environmental problems, and further, as a central energy conversion system in the future age of hydrogen energy, fuel cell technique has been reckoned as one of the cores of new energy technique. Especially, polymer electrolyte fuel cells (PEFC) are tried to be applied as power sources for electric automobiles, power sources for portable apparatuses, and, further, applied to domestically stationary power source apparatuses utilizing electricity and heat at the same time, from the viewpoints of miniaturization and lightening, etc.
A polymer electrolyte fuel cell is generally composed as follows. First, on both sides of a polymer electrolyte membrane having cation conductivity (cation is usually proton), catalyst layers comprising a carbon powder-supported platinum group metal catalyst and a cation-conducting binder comprising a polymer electrolyte are formed, respectively. On the outsides of the catalyst layers, gas diffusion layers as porous materials through which fuel gas and oxidant gas can pass are formed, respectively. As the gas diffusion layers, carbon paper, carbon cloth, etc. are used. An integrated combination of the catalyst layer and the gas diffusion layer is called a gas diffusion electrode, and a structure wherein a pair of gas diffusion electrodes are stuck to the electrolyte membrane so that the catalyst layers can face to the electrolyte membrane, respectively is called a membrane electrode assembly (MEA). On both sides of the membrane electrode assembly, separators having electric conductivity and gastightness are placed. Gas passages supplying the fuel gas or oxidant gas (e.g., air) onto the electrode surfaces are formed, respectively, at the contact parts of the membrane electrode assembly and the separators or inside the separators. Power generation is started by supplying a fuel gas such as hydrogen or methanol to one electrode (fuel electrode) and an oxidant gas containing oxygen such as air to the other electrode (oxygen electrode). Namely, the fuel gas is ionized at the fuel electrode to form protons and electrons, the protons pass through the electrolyte membrane and transferred to the oxygen electrode, the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode, and they react with the oxidant gas to form water. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
Further, in addition to such cation exchange-type fuel cells, anion exchange-type fuel cells using an anion-conducting membrane and an anion-conducting binder (the anions are usually hydroxide ions) are also studied. It is known that in anion exchange-type fuel cells, overvoltage at the oxygen electrode is reduced, and the improvement of energy efficiency is expected. Further, it is said that, when methanol is used as the fuel, methanol crossover wherein methanol passes through the electrolyte membrane between the electrodes is reduced. The constitution of a polymer electrolyte fuel cell in this case is basically the same as in the cation exchange-type fuel cell except that an anion-conducting membrane and an anion-conducting binder are used in place of the cation-conducting membrane and the cation-conducting binder, respectively. As to the mechanism of generation of electric energy, oxygen, water and electrons react at the oxygen electrode to form hydroxide ions, the hydroxide ions pass through the anion-conducting membrane and react with hydrogen at the fuel electrode to form water and electrons, and the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode and react again with oxygen and water to form hydroxide ions. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
As a polymer electrolyte membrane used in cation exchange-type fuel cells, Nafion (registered trade mark of Dupont Co., as is the same hereinafter) which is a perfluorocarbonsulfonic acid-type polymer is generally used. However, Nafion is a fluoropolymer and very expensive. Further, Nafion has a problem that when methanol is used as the fuel, a phenomenon that methanol passes through the electrolyte membrane from one electrode side to the other electrode side (methanol crossover) occurs easily. Further, fluorine-containing polymers contain fluorine and consideration to the environment is necessary at the time of synthesis and disposal.
From such a background as mentioned above, development of cation-conducting polymer electrolyte membranes using a non-fluoropolymer as a base polymer has been tried. For example, there is an example that a polystyrenesulfonic acid-type polymer electrolyte membrane was used in a polymer electrolyte fuel cell developed by General Electric Co. in U.S.A. in the 1950s, but, ones which have been studied heretofore did not exhibit sufficient stability under the environment of operation of fuel cells, and sufficient cell life could not be obtained.
Further, an electrolyte membrane from a heat resistant sulfonated aromatic polyether ketone is proposed (JP-A-6-93114).
Further, a structure wherein by sulfonating the polystyrene block of a block copolymer composed of styrene and a rubber component, the polystyrene block was made to be a cation-conductive channel is proposed. For example, as an inexpensive and mechanically and chemically stable polymer electrolyte membrane, a cation-conductive membrane composed of a sulfonated SEBS (abbreviation of polystyrene-poly (ethylene-butylene)-polystyrene triblock copoymer) is proposed (JP-A-10-503788).
Further, a cation-conductive membrane composed of a sulfonated SiBuS (abbreviation of polystyrene-polyisobutylene-polystyrene triblock copoymer) having an isobutylene skeleton excellent in chemical stability as a rubber component is proposed (JP-A-2001-210336).
On the other hand, as an anion-conductive polymer electrolyte membrane used in anion exchange-type fuel cells, one obtained by radiation graft polymerizing a monomer having an anion exchange group with a fluoropolymer base material is proposed (JP-A-2000-331693).
Further, as an anion-conductive polymer electrolyte membrane, an electrolyte membrane obtained by aminating a chloromethylated aromatic polysulfone-polythioethersulfone copolymer, and making a membrane of the resulting resin is proposed (JP-A-11-273965).
Further, as an anion-conductive polymer electrolyte membrane, an anion exchanger obtained by introducing a quaternary ammonium group in a copolymer composed of a polystyrene block and a polyolefin block is proposed (JP-B-2735693), and an anion exchanger obtained by bonding copolymers themselves each composed of a polystyrene block and a polyolefin block with a polyamine is proposed (JP-B-2996752).